2Review of last lecture What is energy? 3 methods of energy transfer The names of the 6 wavelength categories in the electromagnetic radiation spectrumIntensity of radiation (Stefan-Boltzman law): I=T4Wavelength of radiation (Wein’s law): max = b/TThe wavelength range of Sun (shortwave) and Earth (longwave) raditionThe 11-year solar cycle
3Earth’s orbit around the Sun Distance from the sun accounts for only 7% change in radiation receipt, which is not the cause of seasons.
4Revolution vs. Rotation Revolution: about a body (the earth revolves around the sun; i.e., a year)Rotation: about an axis (The earth rotates about its axis; i.e., a day)So what causes the seasons?!?!
5The 4-seasons is caused by the Earth’s 23 The 4-seasons is caused by the Earth’s 23.5o tilt from the line perpendicular to its orbit plane (toward the sun during summer)
7How the changing orientation of the Earth directly affects the receipt of insolation: Length of Daylight periodAngle at which sunlight hits the surface (“Beam Spreading”)Thickness of atmosphere through which sunlight must travel (“Beam Depletion)
8Length of Daylight Period EquinoxesJune SolsticeDay length increases from equator to pole in summer hemisphereopposite in winterAt least one day of 24-hour sunlight/darkness above Arctic/Antarctic circles, depending on season
9Beam SpreadingBeam spreading: the increase in surface area over which radiation is distributed in response to a decrease of solar angleThe greater the spreading, the less intense the radiation (and vice versa)
11The 4-seasons is caused by the Earth’s 23 The 4-seasons is caused by the Earth’s 23.5o tilt from the line perpendicular to its orbit plane (toward the sun during summer)
12What caused the ice ages and extinction of mammoth?
13Change of Earh’s orbit at longer time scales - The Milankovitch cycles Named after Serbian geophysicist and astronomer Milutin Milankovic, who mathematically theorized that variations in eccentricity, axial tilt and precession of the Earth’s orbit affect Earth’s climateMilutin Milankovic ( )
14Eccentricity: shape of orbit varies between a circle and a thin oval on a 100,000 year cycle Axial tilt with respect to orbit plane: varies between 21.5o and 24.5o on a 41,000 year cyclePrecession: direction of axis varies on a 20,000 year cycle
15Summary The two basic motions of the Earth What causes the four seasons: the Earth’s tilt and the 3 ways it affects the solar insolation (change of length of the day, beam spreading, beam depletion)Change of the Earth’s orbit at longer time scales (Milankovitch cycles): eccentricity, axial tilt, and precession